Magneto-optic recording media are also known by several other names: thermomagnetic media, beam addressable files, and photo-magnetic memories. All of these terms apply to a storage medium or memory element which responds to radiant energy permitting the use of such energy sources as laser beams for both recording and interrogation. Such media modify the character of an incident polarized light beam so that the modification can be detected by an electronic device such as a photodiode.
This modification is usually a manifestation of either the Faraday effect or the Kerr effect on polarized light. The Faraday effect is the rotation of the polarization plane of polarized light which passes through certain magnetized media. The Kerr effect is the rotation of the plane of polarization of a light beam when it is reflected at the surface of certain magnetized media.
Magneto optic recording media have several advantages over known magnetic recording media:
1. The spacing between the medium and recording head is greater, thus reducing potential for contact and
2. Using a pulsed laser beam as the writing means, very high density data storage is possible.
3. With a protective layer on top of a magneto optic layer, the medium is affected less by dust than magnetic media.
In magneto optical recording, data is written into a medium having a preferentially directed remanent magnetization by exposing a localized area (spot or bit) on the recording medium to an electromagnetic or other energy source of sufficient intensity to heat the recording medium above its compensation or Curie point temperature and simultaneously biasing the medium with a magnetic field. Preferably, the energy source is a laser which produces a monochromatic output beam. The magnetic field required to reverse the magnetization of the recording medium varies with the temperature to which the recording medium is brought. Generally speaking for a given material, the higher the temperature, the smaller the required magnetic field coercive force.
The write or record operation for both Curie point and compensation point writing is as follows:
1. The medium is initially in a demagnetized state having about equal numbers of magnetic domains with magnetization oppositely directed and perpendicular to the surface of the film. A domain will herein refer to the smallest stable magnetizable region; although in common usage, a domain is a uniformly magnetized region of any size. The medium may be subjected to a saturation magnetic bias field normal to the surface of the film in order to magnetize all the domains in one direction. Alternatively, a selected area of the medium may be magnetized by exposing said area to a continuous light beam and a small magnetic bias field.
2. A small magnetic bias field oriented perpendicular to the surface or plane of the film, but oppositely directed to the magnetic field applied earlier is applied over the entire thin film medium.
3. With the biasing field in place, a light beam from a radiant energy source such as a laser beam is directed toward a selected location or bit on the film where it causes localized heating of the film to a temperature at or above the compensation temperature. When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The medium, in effect, has a magnetic switching field which is temperature dependent. The magnetic biasing field applied to the irradiated bit selectively switches the bit magnetization, with the bit momentarily near its compensation temperature under the influence of the laser. The momentary temperature rise reduces the bit coercive force.
In the write operation, the write laser beam (e.g. about 8-12 mW) is focused to the desired diameter (e.g. 1.0 microns) onto the surface of the recording medium by an objective lens.
The memory element or recorded bit is interrogated, or read, nondestructively by passing a low-power (e.g. 1-3 mW) beam of polarized light (e.g. a laser beam) through the bit storage site for a sufficiently short time so as not to heat the medium to change its magnetic state. The read laser beam is normally shaped to a circular cross-section by a prism, polarized and focused to some small diameter (e.g. 1.0 microns) onto the recording medium by a lens. When the read beam has passed through the recorded spot, it is sent through an optical analyzer, and then a detector such as a photodiode, for detection of any change or lack of change in the polarization.
A change in orientation of polarization of the light is caused by the magneto-optical properties of the material in the bit or site. Thus, the Kerr effect, Faraday effect, or a combination of these two, is used to effect the change in the plane of light polarization. The plane of polarization of the transmitted or reflected light beam is rotated through the characteristic rotation angle .theta.. For upward bit magnetization, it rotates .theta. degrees and for downward magnetization -.theta. degrees. The recorded data, usually in digital form represented by logic values of 1 or 0 depending on the direction of bit magnetization, are detected by reading the change in the intensity of light passing through or reflected from the individual bits, the intensity being responsive to the quantity of light which is rotated and the rotation angle.
Erasure can be accomplished by simply writing new information over old portions of the medium or by simply exposing any given bit with a laser beam of sufficient intensity and then cooling that bit in the presence of a magnetic field in the direction of the initially applied magnetic field. The entire storage medium can be erased by providing a large magnetic bias field in the original saturation direction which does not require a laser beam. Generally, in the recording process, the external biasing magnetic field is applied by a magnet set above or behind the magneto optic medium, and in the erasing process, the magnet is reversed in direction.
The signal-to-noise ratio (SNR) or carrier-to-noise ratio (CNR) of an erasable magneto optic medium is proportional to .theta..sqroot.R, where R equals reflectivity of the medium and .theta. is the angle of rotation. Forty-five decibels in a 30 kHz band width is generally considered the minimum CNR acceptable for direct read after write (DRAW) media. The speed at which the bits can be interrogated and the reliability with which the data can be read depends upon the magnitude of the magneto-optical properties, such as the angle of rotation, of the thin film and upon the ability of the interrogation system to detect these properties. An increase in the angle of rotation .theta. usually results in an increase in CNR.
For purposes of this discussion, the noise floor or noise level is measured at the average noise level.
The main parameters that characterize a magneto optic material are the angle of rotation, the coercive force (H.sub.c) the Curie temperature and the compensation point temperature. The medium is generally comprised of a single element or multicomponent system where at least one of the components is an amorphous metal composition. Binary and ternary compositions are particularly suitable for these amorphous metal alloys. Suitable examples would be rare earth-transition metal (RE--TM) compositions, such as: Gadolinium-cobalt (Gd--Co), Gadolinium-iron (Gd--Fe), Terbium-iron (Tb--Fe), Dysprosium-iron (Dy--Fe), Gd--Tb--Fe, Tb--Dy--Fe, Tb--Fe--Co, Terbium-iron-chromium (Tb--Fe--Cr), Gd--Fe--Bi (Bismuth), Gd--Fe--Sn (Tin), Gd--Fe--Co, Gd--Co--Bi, and Gd--Dy--Fe.
Japanese patent publication No. 56/143547 discloses a magneto optic medium of the type just discussed. It comprises a thin film of gadolinium-terbium-iron alloy in a ratio of 0.24/0.18/1 which film is more than 1000 angstroms thick when using the Kerr effect and 500 to 800 angstroms thick when using the Faraday effect. The film of this patent also has a 5400 angstrom thick glass (silicon dioxide) film on top of the Gd:Tb:Fe film.
The magneto optic amorphous thin films can be fabricated by known thin film deposition techniques, such as sputtering, evaporation and splat cooling. In splat cooling a hot liquid of the film constituents is incident on a cool surface where they are quenched and solidified rapidly to form an amorphous bulk film. Generally, no matter what deposition rate is used, the substrate temperature must be less than that at which crystallization occurs in order to provide amorphous magnetic materials.
The preferred process for thin film deposition is sputtering. Typical known sputtering conditions for amorphous thin films are: initial vacuum less than 1.times.10.sup.-5 Torr; sputtering pressure of from 3.times.10.sup.2 to 2.times.10.sup.-2 Torr; pre-sputtering of a sputtering source of material to clear the surface thereof; substrate temperature of 30.degree. to 100.degree. C.; and an argon partial pressure.
In the cathodic sputtering process, argon gas ions bombard the solid alloy target cathode in the sputtering chamber dislodging metal atoms by transferring the momentum of the accelerated ions to the metal atoms near the surface of the target. The cathode is said to glow, and the mass of ionized gas between the cathode and the anode is a plasma. The substrate is placed at the anode, and the metal alloy atoms traverse the space between the anode and cathode to deposit or condense on the substrate.